1. Field of the Invention
The present invention relates to a packet format of audio data for transmitting digital audio data in real time and a communication system using the same packet format, in particular to a packet format of audio data transmitted between audio and visual devices (referred to as AV devices hereinafter) and a communication system using the same packet format.
2. Description of the Related Art
AV devices adopting an HDMI (High Definition Multimedia Interface) have been in widespread use in the market. In this case, the HDMI is an interface standard for next-generation digital televisions for transmitting an uncompressed baseband video signal and a digital audio signal via one wired transmission cable. Conventionally, it has been required to use a plurality of cables for respective signals such as a video signal and an audio signal, in order to connect a plurality of AV devices to each other. However, since only one cable connection is required in the HDMI, there is such an advantageous effect that a quite simple wiring can be provided. In addition, since data are transmitted digitally in the HDMI, there is such an advantageous effect that data transmission with large noise resistance and high image quality can be provided. Further, since control signals can be transmitted bi-directionally, it is possible to control a television set and a DVD player so as to cooperate with each other, or it is possible to construct a home theater by a surround loudspeaker and a large-screen display and control an entire system of the home theater. In the HDMI, since high quality contents can be transmitted, an HDCP (High-bandwidth Digital Content Protection System) is used as a content protection method for preventing illegal use and illegal copying of provided contents. In the HDCP, there are defined device authentications at a transmitter side and a receiver side, a key sharing system for the authentications, and an encrypting method for the contents to be transmitted.
FIG. 25 is a diagram showing data of audio packets for use in a communication system according to a prior art and compliant with the HDMI. Referring to FIG. 25, an operation for transmitting an audio data packet will be described below.
Referring to FIG. 25, an audio packet includes a packet header 71 of audio data and a packet payload 72 of the audio data. In this case, the packet header 71 includes a layout bit 73, a sample present field 74, a B field 75 and a sample flat field 76. In addition, the packet payload 72 of the audio data includes L channel sample data 77a to 77d, R channel sample data 78a to 78d, R channel status fields 79a to 79d, and L channel status fields 80a to 80d. 
Transmission using the packet format of the audio data configured as described above will now be described.
A source device and a sink device are connected to each other via an HDMI cable, and video data is transmitted from the source device to the sink device. In addition, the audio data is time-division multiplexed, and thereafter, transmitted during the blanking intervals of the video data. Each packet of the audio data includes the packet header 71 and the packet payload 72. The packet header 71 includes the layout bit 73, the sample present field 74, the B field 75, and the sample flat field 76. In addition, the packet payload 72 includes the L channel sample data 77a to 77d, the R channel sample data 78a to 78d, the R channel status fields 79a to 79d, and the L channel status fields 80a to 80d. In this case, the layout bit 73 represents a configuration of audio samples included in the payload 72 of the packet. In addition, the sample present field 74 represents whether or not an audio sample is included in a predetermined position of the packet payload, the B field 75 represents whether or not a first frame compliant with an IEC60958 Standard (in this case, IEC is an abbreviation of International Electrotechnical Commission) is included, and the sample flat field 76 represents whether or not the audio sample is a flat line sample.
First of all, such a case is described below where the layout bit 73 is 0. In this case, audio samples for up to two channels are allocated in total to the L channel sample data 77a to 77d and the R channel sample data 78a to 78d
TABLE 1L channel sample data 77a = Audio sample 0 for channel 1;L channel sample data 77b = Audio sample 1 for channel 1;L channel sample data 77c = Audio sample 2 for channel 1; andL channel sample data 77d = Audio sample 3 for channel 1.R channel sample data 78a = Audio sample 0 for channel 2;R channel sample data 78b = Audio sample 1 for channel 2;R channel sample data 78c = Audio sample 2 for channel 2; andR channel sample data 78d = Audio sample 3 for channel 2.
In addition, the R channel statuses 79a to 79d and the L channel statuses 80a to 80d are related to the R channel sample data 78a to 78d and the L channel sample data 77a to 77d, respectively, and each of the R channel statuses 79a to 79d and the L channel statuses 80a to 80d includes a V (valid bit), an U (User Data bit), a C (Channel Status), and a P (Parity) compliant with the IEC60958 Standard.
Next, such a case is described below where the layout bit 73 is 1. In this case, audio samples for up to eight channels are allocated in total to the L channel sample data 77a to 77d and the R channel sample data 78a to 78d.
TABLE 2L channel sample data 77a = Audio sample 0 for channel 1;L channel sample data 77b = Audio sample 0 for channel 3;L channel sample data 77c = Audio sample 0 for channel 5; andL channel sample data 77d = Audio sample 0 for channel 7.R channel sample data 78a = Audio sample 0 for channel 2;R channel sample data 78b = Audio sample 0 for channel 4;R channel sample data 78c = Audio sample 0 for channel 6; andR channel sample data 78d = Audio sample 0 for channel 8.
By using the above-mentioned packets of audio data, the digital audio data can be transmitted from the source device to the sink device in real time. For example, the audio data packet is shown in the following Patent Document 1 and Non-Patent Document 1.
Patent Document 1: Japanese patent laid-open publication No. JP-2005-295394-A.
Non-Patent Document 1: High-Definition Multimedia Interface Specification Version 1.3a, 2006 Nov. 10.
AUDIO SPECIFICATIONS, Version 1.0, Annex B″, DVD Forum, March 1999.